1. Field of the Invention
The present invention generally relates to a lamp driving system. More particularly, the present invention relates to a multi-lamp driving system in the application of the backlight module of a liquid crystal display.
2. Description of the Related Art
A discharge lamp used to backlight an LCD panel such as a cold cathode fluorescent lamp (CCFL) has terminal voltage characteristics that vary depending upon the immediate history and the frequency of a stimulus (AC signal) applied to the lamp. Until the CCFL is struck or ignited, the lamp will not conduct a current with an applied terminal voltage that is less than the strike voltage, e.g., the terminal voltage must be equal to or greater than 1500 Volts. Once an electrical arc is struck inside the CCFL, the terminal voltage may fall to a run voltage that is approximately ⅓ the value of the strike voltage over a relatively wide range of input currents. For example, the run voltage could be 500 Volts over a range of 500 microAmps to 6 milliAmps for a CCFL that has a strike voltage of 1,500 Volt. Usually, the CCFL is driven by AC signals having frequencies that range from 30 KiloHertz to 100 KiloHertz.
The discharge lamp exhibits a negative impedance characteristic that the equivalent impedance is decreased upon an increase of input power. Therefore, a circuit for providing the lamp with power, such as an inverter, should be configured with a controllable alternating current power supply and a feedback loop for monitoring the current flowing through the lamp to ensure stable operation and make load regulation as well.
Referring to FIG. 1, a conventional lamp driving system is schematically depicted. The system of FIG. 1 has only one feedback loop used for controlling the total current flowing through a lamp or lamps, but not used for controlling or balancing those currents flowing througheach lamp. If the current through one lamp is significantly larger than others, the lamp will be shortened in lifetime and the LCD panel will be degraded in brightness uniformity.
Referring to FIG. 2, another conventional lamp driving system is schematically depicted. However, the system of FIG. 2 is configured with two sets of control circuits resulting in an increase of cost and space.
Referring to FIG. 3, further another conventional driving system is schematically depicted. However, the system of FIG. 3 has two transformers also resulting in an increase of cost and space. Moreover, the transformers are configured with secondary coils connected in parallel to be adverse to high-voltage processing.
It is therefore an object of the present invention to provide a lamp driving system for controlling the balance of currents at load end, which can be extensively applied to a system with single feedback loop and multiple loads.
It is another object of the present invention to provide a lamp driving system with cost efficiency, compact space and simplified manufacturing.
It is further another object of the present invention to provide a lamp driving system for controlling the balance of currents precisely.
To achieve aforementioned objects, the present invention provides a multi-lamp driving system comprising: an inverter for generating an AC power, a lamp set having a first lamp and a second lamp, and a balancing controller coupled with the inverter and the lamp set for balancing currents flowing through the first lamp and the second lamp. The balancing controller comprises: a first load coupled with the first lamp and the inverter, a second load coupled with the second lamp and the inverter, and a third load coupled with the first load and the second load, wherein the impedance ratio of the third load to the first load is negative.
Moreover, the present invention provides a multi-lamp driving system comprising: an inverter for generating an AC power, a lamp set having a plurality of lamps, and a balancing controller coupled with the lamp set and the inverter for balancing currents flowing through the plurality of lamps. The balancing controller comprises: a plurality of loads, each of which is coupled with one of the plurality of lamps and the inverter; and a load choke coupled with the plurality of loads to balance currents flowing through the plurality of lamps.